The present invention relates to opening devices for closures that use a ring pull or tab to tear a foil seal.
The present invention addresses the technical problem of minimising the effort needed to open a container closure. It is important to keep the force required to open containers to a minimum in order to reduce the risk of spillage during opening and to enable frail users to open the closure.
When a ring pull device is used to open a container, the force is transmitted from the finger in the pull ring to a connected part that initiates the opening. In the case of a ring pull for opening a metal can, the tab or pull ring is connected to a pointed nib, which acts on a frangible portion of a seal. The nib concentrates the force applied by the user at a specific point in order to reduce the force that the user needs to apply. Such a construction is described in GB-A-1 262 272 (Cookson).
However when a ring pull is used to tear a plastics seal, it is typically connected to a removable part within a spout by means of one or more legs. See for example GB-A-2 377 701 (Spreckelsen McGeough Ltd) U.S. Pat. No. 4,682,702 (Gach 1) or U.S. Pat. No. 4,815,618 (Gach 2). In Gach 1 a spiral weakening groove is provided in the removable part, which takes the form of a sealing disc that provides the sole seal across an opening in the spout. The spiral groove divides the disc into a tear strip. The legs of the pull ring are attached to the tear strip at the periphery of the disc. Pulling up on the ring starts the separation of the tear strip along both sides of the strip opening the closure. Gach 1 is primarily designed for tamper evidence and ease and obviousness of separation is important for this reason. The pressure required to initiate the tear is determined solely by the depth of the groove. The need to tear a foil creates a further technical problem.
Spreckelsen McGeough Ltd and Gach 2 disclose a closure comprising:                a spout defining an opening,        a removable plastics part connected to the spout by means of a frangible region,        a pulling device connected to the removable part by means of a leg, and        a foil secured to the removable part and the spout to form a seal across the opening.        
With this type of closure the removable part is typically a circular disc. An upward pulling force applied by the user during opening is transmitted to the foil. The force applied by the pulling device is typically distributed over a large arc of the frangible region extending in both directions away from the mounting of the device and also inwardly towards a centre of the removable part. A tear is initiated when the pressure on the foil reaches a tearing threshold or failure modulus, which depends on the nature of the foil. When this type of closure is used with foils having a polypropylene (as taught by Gach 2) or PET compatible layer—as opposed to a polyethylene compatible layer as taught in Spreckelsen McGeough Ltd—the threshold opening pressure is relatively high. This makes this type of closure difficult to open when used with polypropylene and PET containers or plastics laminated with metals or plastics laminated with paperboard containers. This can be understood more clearly by reference to FIG. 1 in which
FIG. 1a shows a top view of a circular foil to be torn by pulling at the point marked x;
FIG. 1b shows a side view illustrating the application of an opening Force F to the foil;
FIG. 1c shows an element of the foil;
FIG. 1d shows the cross-sectional area A of the foil;
FIG. 1e shows the location of al relative to the circular foil;
FIG. 1f shows a diagrammatic section through a ring pull attached to a foil at the point of tear;
FIG. 1g shows a schematic diagram of FIG. 1f; and
FIG. 1h show how al increases with increasing diameter of a circular foil.
      Stress    =                  Force                  Cross          ⁢                      -                    ⁢          sectional          ⁢                                          ⁢          area          ⁢                                          ⁢          of          ⁢                                          ⁢          foil                    =              F        A                  Strain    =                            Elongation          ⁢                                          ⁢          of          ⁢                                          ⁢          foil                          Original          ⁢                                          ⁢          length                    =              e        l                  Modulus    =                  Stress        Strain            =                        F          ·          l                          A          ·          e                    
The failure stress is when the sample fails as the force (F) is applied across area (A).
For a round foil membrane the area A is calculated by the thickness (t) of the foil times the arc length (al) over which the force is acting. For a given thickness therefore F∝al. The larger the diameter—the larger the arc length over which the force has to act to tear the foil—the larger the force required to reach the same failure stress.
This technical problem of achieving an opening pressure when the pulling force is distributed over a large area increases with the size of the removable part, making it extremely difficult to open wide mouthed PET or polypropylene or plastics laminated metal or paperboard containers and even wide mouth polyethylene containers with this type of closure. This problem can lead to delaminating of the foil and/or snapping of the ring pull.
Once the tear has been initiated, the foil is then torn in both directions away from a base of the leg around the circumference of the disc. The greater the circumference the more upward force is required in order to resolve sufficient force in both directions in the foil bridging the frangible region in order to create and propagate tears running both ways round the disc.